The corpus callosum is an important cerebral commissure that connects the cerebral hemispheres. It takes its shape from anterior to posterior, with the exception of the rostrum, which is the last one to get formed. So, in case of partial agenesis, both the splenium and the rostrum are not evident. A hypoplastic or atrophied corpus callosum results from a cerebral insult after its formation (around 18 weeks of gestation).

The agenesis of the corpus callosum can be diagnosed by ultrasound. MRI is though essential to a diagnostic confirmation of more dubious cases. The diagnosis is based mainly on indirect signals, such as: parallelism of the lateral ventricle, ascension of the third ventricle, dilatation of the occipital horn of the lateral ventricle, absence of the cingulate gyros with gyro verticalis and presence of bands of Probst, formed by fibers that fail to reach the contra lateral hemisphere and that bend subsequently causing indentation of the anteromedial wall of the frontal horn (Table 1).

Table 1: Agenesis of the Corpus Callosum

Imaging findings:

ascension of the third ventricle, splitting upwards into the inter hemispheric fissure (cleft), with or without inter hemispheric dorsal cyst;

radial disposition of gyro;

parallelism of lateral ventricles;

Bands of Probst;

dilatation of the occipital horn of the lateral ventricle

Sonigo et al. (1998) studied 50 fetuses with agenesis of the corpus callosum (postnatally confirmed) either by ultrasound transfontanelle or by anatomicopathological studies. The ultrasonography detected 66 percent of this type of agenesis as well as 16 percent of associated cerebral anomalies. However, MRI detected 99 percent of agenesis of the corpus callosum and 61 percent of associated malformations. Such malformations were anomalies of neuronal migration, anomalies of the posterior foss, cortical atrophy, hypoplasia of the brain stem and a diffuse gliosis. Garel et al. (1998) also studied 20 fetuses with agenesis of the corpus callosum. 95 percent of cases were diagnosed by MRI, while 80 percent were diagnosed by ultrasound. MRI could identify 15 associative anomalies out of 33 cases, whereas ultrasound could identify 5 associative anomalies. Among the anomalies, neuronal migration, gyro, posterior fossa and inter hemispheric cysts (Table 2).